The present invention relates to electronically scanned antennas, and more particularly to such antennas in which beam scanning in one dimension is accomplished by means of simultaneous frequency and phase control.
Both frequency and phase scanned antennas are well known in the art. Frequency scanned antennas have the advantages of simplicity and low cost; however, they do have certain drawbacks, including relatively large power losses in the feedlines, especially at higher radio frequencies, and narrow instantaneous bandwidth. Phase scanned antennas, on the other hand, provide for wide bandwidth, low power losses, and have the capability of radiating at multiple frequencies in a given beam direction. However, the conventional phase scanned antenna is quite expensive due to its high complexity and need for many expensive phase shifters.
Heretofore, conventional frequency scanned antennas have employed single continuous folded feedlines, such as those described in U.S. Pat. No. 3,434,139 for "Frequency Controlled Scanning Monopulse Antenna" by J. A. Algeo and U.S. Pat. No. 3,419,870 for "Dual Plane Frequency Scanned Antenna Array" by S. H. Wong. The continuous folded feedline, commonly referred to as a serpentine feed, is well known in the art, and is coupled to a plurality of radiating elements. The spacing between adjacent radiating elements and the length of the feedline between adjacent elements determine the spatial beam steering capability of the antenna in response to the frequency scan. In general, the element spacing should be sufficiently small to avoid grating lobes. For a better understanding of grating lobes and their effect upon antenna performance, see U.S. Pat. No. 3,825,928 for "High Resolution Bistatic Radar System" or U.S. Pat. No. 3,842,417 for "Bistatic Radar System," both by F. C. Williams.
For example, if the element spacing is chosen to be 0.65.lambda., a phase change of approximately .+-.120.degree. between adjacent elements is required to produce .+-.30.degree. of spatial beam motion. This result is determined from the equation EQU .PHI.=(2.pi.d/.lambda.) sin .theta.
where .PHI. is the phase between adjacent elements, d is the element spacing, .lambda. is the wavelength of the energy and .theta. is the scan angle as measured from broadside.
The length s of the feedline between adjacent elements may be determined from the equation EQU .phi.=2.pi.s/.lambda..sub.g
where .phi. is the phase between adjacent elements and .lambda..sub.g is the wavelength of the energy in the feedline. To simplify calculating s, the dispersion in the feedline is assumed constant over the frequency bandwidth. If the center frequency is f.sub.1, and the end of the band is at frequency f.sub.2, the total frequency excursion is .+-..vertline.f.sub.2 -f.sub.1 .vertline.. Let k be the ratio between feedline and free-space wavelengths. Then EQU .lambda..sub.g1 =k.lambda..sub.1 =(kc/f.sub.1) EQU .lambda..sub.g2 =k.lambda..sub.2 =(kc/f.sub.2)
and ##EQU1## where c is the speed of light, and .lambda..sub.1 and .lambda..sub.2 are the free space wavelengths corresponding to f.sub.1 and f.sub.2 respectively.
In a practical application utilizing an X-band radar operating at 10 GHz, let f.sub.2 -f.sub.1 =500 MHz; the speed of light is c=3.times.10.sup.8 m/sec; and let k=1.4. Accordingly, s=0.27 meters for .PHI..sub.2 -.PHI..sub.1 =120.degree.. A typical X-band antenna might be as much as 10 meters long. Using the 0.65.lambda. element spacing, there are approximately 512 radiating elements, and the total length of the feedline is about 140 meters.
The 140 meter feedline length corresponds to a theoretical bandwidth of 700 KHz and a practical bandwidth of about 70 KHz. Such a bandwidth results in a range resolution of about 1400 meters, which greatly reduces the applications for which the radar is useful.
The 140 meter length of feedline also results in a theoretical power degradation at 10 GHz of 14 dB over the length of the feedline. It has been found in actual practice, however, that this power loss is on the order of 30 dB. Typical power losses at 20 GHz would be on the order of 50 dB. Additionally, as present radars now operate at millimeter wavelengths, above 35 GHz or so, feedline losses are markedly higher than in the example hereinabove and may be on the order of 90 to 100 dB.
An additional problem associated with a conventional frequency scanned antenna is its susceptibility to second-go-round returns. Second-go-round returns occur in a radar system, for example, when the system transmits a first pulse of energy at one frequency at time t.sub.1, and a second pulse of the same frequency at a time t.sub.2, and then a return pulse is received from a target at time t.sub.3. Since all pulses are at the same frequency, the system is incapable of determining whether the return pulse is from the first or second transmitted pulse. Consequently, the system cannot determine the range of the target.
To circumvent the problems of feedline power losses, narrow bandwidths, and second-go-round returns, one prior art solution has been to incorporate individual phase shifters for each radiating element or use multi-bit phase shifters driving separate pluralities of elements, thus eliminating the serpentine feedline. The result is a phase scanned antenna as contrasted to the frequency scanned antenna above.
Typical of the phase scanned antenna is U.S. Pat. No. 4,045,800 for "Phase Steered Subarray Antenna" by R. Tang et al. This antenna incorporates a corporate feedline or power divider which operates to divide power equally at its outputs as well as providing equal length electrical paths therethrough. The corporate feedline feeds power to a plurality of 360.degree. multi-bit phase shifters which in turn feed either one-bit or two-bit phase shifters which finally apply power to individual radiating elements.
Although power losses are reduced and bandwidth is much higher since no serpentine feedline is used, the overall complexity and cost of the system has increased. Considering the example hereinabove having 512 radiating elements, 512 phase shifters and associated drivers are necessary to make the antenna operational. In addition, it is necessary to further incorporate a computer to provide overall control of the phase shifters.